In the techniques related to automobiles, mass production is the mainstream. Recently, however, the vehicle types are varied, with model change being effected frequently. In this context, from the viewpoint of cost, delivery time, requisite precision, etc., die production through casting is widely performed to produce a die for forming a relatively large product of a plastic material to be used as an automobile interior component, for example, urethane padding (seat pad) for an automobile seat or the like.
As a method for producing such a die, a method including the following steps has been conventionally adopted:
(1) Master model body cutting step, in which a master model body is formed through manual processing or NC processing by using a synthetic resin type material such as polyurethane or polystyrene;
(2) Surface treating step, in which the surface of the formed master model body is treated to enhance the surface precision thereof;
(3) Master model making step, in which a master model is made by providing the master model with a flange, an auxiliary rib, a thick sheet which is for furnishing mold thickness, and others, so that the master model is used for producing a die by casting;
(4) Mold preparing step, in which a mold is prepared by packing sand by the use of the master model;
(5) Die casting step, in which a die is cast by pouring molten die material (e.g., aluminum alloy) into the mold; and
(6) Finishing step, in which the casting die is finished by electric discharge machining or the like.
The above-mentioned conventional method requires a great number of working days. It takes approximately one month to obtain the die. In this context, a conventional improved method including the following steps has been proposed with a view to shortening the working days (Patent Document 1):
(1) Automatic processing preparation step, in which processing data in conformity with an article to be produced is prepared and inputted into an automatic processing machine (e.g., an NC machine) to record the data;
(2) Master model cutting and producing step, in which a master model is produced by cutting a master model body with an automatic processing machine;
(3) Sand mold preparation step, in which a sand mold is prepared by packing casting sand into the master mold placed in an outer frame and solidifying the sand to an appropriate hardness with bond;
(4) Die mold preparation step, in which a die mold is prepared by packing casting sand in the prepared sand mold, providing it with a gate, and solidifying the sand to an appropriate hardness by bond;
(5) Die casting step, in which a die is cast by pouring a predetermined molten die material (e.g., aluminum alloy) into the die mold; and
(6) Finishing and assembling step, in which the casting die is finished and assembled.
In the above-mentioned die producing process in which the above-mentioned steps (1) through (6) are combined, the master model cutting and producing step is characterized in that the master model body is divided into several portions according to the degree to which the die is complicated, the several portions of the master model body are combined together again into an integral unit, and the integral unit is cut and processed.